Bifold doors are doors that fold against each other "accordion style" as opposed to doors that swing open. They are frequently used in homes and apartments for closet doors because they take less space than a traditional "swing" door".
Bifold doors have a pivot pin inserted in the stile, (the vertical board in the edge of a door), at the top and bottom of the part of the pair of doors installed nearest the vertical door jam. When the bifold doors are installed those pivot pins are inserted in pivot pin bearings at the top and bottom of the door jam. A second pin or roller is inserted in the top of the door farthest from the first door jam. This pin or roller is inserted up into an inverted "U" channel affixed to the top of the door jam. The pair of bifold doors are joined together by a hinge along their common vertical edges.
As the bifold doors are drawn open, the pivoted ends of the doors rotate on the pivot pins. This cause the hinged edge of the pivot pin door to trace an arc. Since the two doors are hinged together, the hinge pulls the edge of the second door back, towards the pivot pin jam. This causes the distant end of the pair of doors to be drawn back toward the pivoted ends. The distant end guide pin or roller inserted in the track, securely guides the distant end of the doors back toward the door jam. As the distant end is drawn back, the pair of doors fold against each other along the hinge which joins them.
As installed, the bifold doors pair are cantilevered out from the pair of pivot pins. The weight of the extended pair of doors pulling down exerts a tension across the grain of the stile in the top of the pivot pin door. The stile of the pivot pin door has already been weakened by drilling a hole in it to insert the pivot pin. Eventually, due to drying wood or use of excessive force on the door, the stile of the door is pulled open along the grain, almost always at the point of the drilled hole for insertion of the pivot pin. When that happens, the door opens up, the pivot pin is released, and the whole system becomes unusable.
Previously, the only way to repair the problem was to replace the door. This involved finding a matching door, finding hardware that was compatible with the hardware that was installed in the door jam and the existing door, fitting the new door to the existing door, fitting the new door to the existing opening, installing the hardware, trying to match the paint color of the new paint with the paint on the existing door and then painting the new door. At best the whole process was a labor intensive, expensive, time consuming undertaking. Frequently, a matching door or compatible hardware cannot be found and all the doors, hardware and tracks have to be replaced.